The invention relates to aqueous dispersion adhesives based on aqueous polyurethane-polyurethane urea dispersions, to a process for their preparation and to the use of the dispersion adhesives in the production of adhesive composites.
The preparation of aqueous polyurethane-polyurethane urea dispersions is known. When such dispersions are used as adhesives for the bonding of substrates, the heat activation method is frequently employed. In this method, the dispersion is applied to the substrate and, when the water has evaporated completely, the adhesive layer is activated by heating, for example with an infra-red radiator, and brought into a sticky state. The temperature at which the adhesive film becomes tacky is referred to as the activation temperature. As low an activation temperature as possible is generally desired, because a disadvantageously high outlay in terms of energy is required at high activation temperatures and manual joining becomes difficult to impossible.
Adhesives based on aqueous polyurethane-polyurethane urea dispersions, which are suitable for application of the heat activation method, are described in U.S. Pat. No. 4,870,129. According to that specification, it is possible by using specific mixtures of diisocyanates to obtain aqueous polyurethane-polyurethane urea dispersions by the acetone process, and the films obtainable therefrom can readily be activated at 40° to 80° C.
Likewise, EP-A 0 304 718 describes the preparation of aqueous polyurethane-polyurethane urea dispersions which permit the production of films which can readily be activated. This is achieved by the use of specific amino compounds as chain extenders. The amino compounds are primary and/or secondary monoamino compounds, optionally in admixture with primary and/or secondary diamino compounds having a mean amino functionality of from 1 to 1.9. The equivalent ratio of NCO groups in the NCO prepolymer to the total amount of hydrogen atoms active towards isocyanates is from 0.5:1 to 0.98:1. Mixtures of diamino and monoamino compounds are preferably used. DE-A 2551094 describes polyurethanes which are dispersible in water and have lateral polyalkylene oxide-polyether chains and ionic groups (sulfonate, carboxylate or ammonium groups) and which can be used inter alia also as adhesives.
A fundamental disadvantage of these dispersion adhesives described in the prior art is their inadequate initial heat resistance. In addition, they exhibit inadequate final heat resistance in the case of one-component adhesive bonding, one-component within the scope of the present invention meaning that an additional polyisocyanate compound is not added as crosslinker component prior to application.
A further method of preparing aqueous polyurethane-polyurethane urea dispersions which are suitable as an adhesive in particular according to the heat activation method is disclosed in DE-A 101 52 405. In this specification, aqueous polyurethane-polyurethane urea dispersions can be obtained by using specific polyester polyols comprising aromatic metal sulfonate groups. Films produced therefrom by removing the water can readily be activated even at 50 to 60° C. However, these polyesters comprising aromatic metal sulfonate groups can be obtained only with difficulty or are very expensive owing to the dicarboxylic acids containing metal sulfonate or sulfonic acid groups which are necessarily to be used as raw materials.
DE-A 10 2004 023 768 discloses a further method of preparing polyurethane-polyurethane urea dispersions which are suitable as adhesives having good initial heat resistance. However, the content of up to 7.5 wt. % of an external emulsifier adversely affects the possible uses of these adhesives, because they cause high hydrophilicity and sensitivity of the products to water. In addition, the adhesion and bonding force can be adversely affected by migration effects of the emulsifier, which is not chemically bonded.